The present invention relates to a Faraday rotator device capable of changing a polarized wave plane of a beam of light by changing current, and an optical switch comprising such a Faraday rotator device for switching the propagation direction of the light between two directions.
When a beam of light generated by a light source such as a semiconductor laser is caused to propagate through two optical fibers alternately, an optical switch as shown in FIG. 10 is conventionally used. In this optical switch, a prism 13 is mechanically moved back and forth to change the optical path of an incident light beam 5 from "a" to "b," and from "b" to "a." Herein 12a and 12b each respectively denote a total reflection prism. However, though a high extinction ratio is achieved in this apparatus, a mechanically movable portion (prism 13) is liable to be worn by a long period of use, leading to deviation in optical axis and thereby lowering the reliability of the apparatus.
To solve this problem, an optical switch as shown in FIG. 11 which utilizes a magneto-optic effect was proposed (Japanese Utility Model Publication Nos. 63-144614 and 63-144615).
This optical switch utilizes the principle of an optical isolator. By flowing a current through a wire winding 3 in one direction, a magnetic field 7a is generated, whereby a Faraday rotator 2 is magnetized. An incident light beam 5 passing through a first polarizer 6a goes into the Faraday rotator 2 in which the polarized wave plane of the light is rotated by 45.degree., and the light beam goes into a second polarizer 6b. Since the polarization direction of the second polarizer 6b is in alignment with the polarized wave plane of the light, the light beam can pass through it as shown by "a" in FIG. 11.
In this state, when the direction of the current is reversed, the magnetization direction of the Faraday rotator 2 is reversed to 7b, and the polarized wave plane of the incident light beam 5 becomes perpendicular to the polarization direction of the second polarizer 6b. Accordingly, the light beam cannot pass through the second polarizer 6b, and comes out in a perpendicular direction as shown by "b." By connecting an optical waveguide such as an optical fiber in this direction, the light beam can be caused to propagate in the other direction "b."
In this optical switch, the incident light beam 5 can be electromagnetically controlled in its propagation direction "a" or "b" by reversing the magnetization direction of the Faraday rotator 2.
However, in the conventional optical switch shown in FIG. 11 utilizing the function of an optical isolator, extremely high electric power is needed to reverse the magnetization direction of Faraday rotator 2, because the Faraday rotator device constitutes an open magnetic circuit.
Proposed as one method for solving this problem is a system utilizing a Faraday rotator 2 made of a thin YIG single crystal layer formed on a substrate 9 shown in FIG. 12 (1983 General Meeting of the Communications Association, 2276, and 1983 General Meeting of the Communications Association, S13-14). In the optical switch shown in FIG. 12, a magnetic yoke 1 having open ends is used, and a thin layer-type Faraday rotator 2 is disposed between the open ends of the magnetic yoke 1 to provide a substantially closed magnetic circuit. A wire winding 3 is wound around the ring-shaped magnetic yoke 1.
In this system, since the thin layer-type Faraday rotator 2 is used, the demagnetizing field is extremely small. Accordingly, the magnetization direction of the Faraday rotator 2 can be reversed with an extremely low magnetic field, namely with an extremely small current.
However, in this system, the beam diameter of the light which can be used is inherently limited due to the use of the thin layer, so that the applications of this system are inevitably limited.
Thus, in such conventional optical switches utilizing a magneto-optic effect, a high current is needed to reverse the magnetization direction of the Faraday rotator when a light having a large beam diameter is intended to be passed. In other words, when only a small current is used, a light having a large beam diameter cannot be passed.